Process and device for evaporating liquids, for example black liquor from cellulose cooking, which contain solid and dissolved substances

ABSTRACT

The present invention relates to a process and a device for evaporating liquids, for example black liquor from cellulose cooking, which contain solid and dissolved substances which are to be concentrated. The liquid is heated to close to boiling temperature in a heat exchanger ( 1 ) of the falling-film type. The process is characterized in that the pressure on the liquid side in the falling-film heat exchanger ( 1 ) is kept so high that boiling does not take place in this heat exchanger and in that the evaporation takes place, by means of flashing, in a separate expansion tank ( 7 ) that has a lower steam.

PRIOR APPLICATION

This application is a U.S. national phase application based onInternational Application No. PCT/SE01/02559, filed 20 Nov. 2001,claiming priority from Swedish Patent Application No. 004265-5, filed 22Nov. 2000.

TECHNICAL FIELD

The present invention relates to a process and a device for evaporatingliquids, in particular black liquor from cellulose cooking. This liquidcontains organic material which has been released from the wood, inparticular lignin residues in the form of released lignin, hemicelluloseand chemicals which have been added on account of the cooking. Thesubstances are present in the form of solid particles and as dissolvedsubstances. The purpose of the evaporation is to concentrate thedissolved and solid substances so that they can subsequently becombusted in what is termed a recovery boiler, if the liquid is a blackliquor.

STATE OF THE ART

It is usual, and has been known for a very long time, to evaporateliquids which contain organic substances or chemicals in order to beable to combust the residue, and thereby generate heat, and to be ableto recover the inorganic chemicals. This technique is particularly welldeveloped in association with evaporating black liquor from cellulosecooking. Such an evaporation takes place in several stages in severalevaporation appliances termed effects, with the most dilute black liquorbeing fed into one effect in order to be fed onwards in subsequenteffects, with the concentration of the black liquor increasing as theblack liquor is evaporated using steam which is frequently conducted ina combination of cocurrent and counter-current directions relative tothe direction taken by the liquor. An example of a standard coupling ofthe effects is that the liquor goes in the sequence 456321 if the steamhas the sequence 123456. The effects are normally numbered by thedirection of flow of the steam determining the number sequence. Thesteam which is released from the liquor in one effect is used forevaporation in the subsequent effect. In the case of the highestconcentrations, the pressure on both the steam side and the liquor sideincreases as the concentration increases, which means that thetemperatures increase the more concentrated the liquor becomes. Thefinished product usually has a dry substance content of 70-80% and cantherefore be combusted in a recovery boiler.

The appliance which is used for such an evaporation normally consists oftall, vertical cylinders which are provided internally with a heatingsurface consisting of either pipes or lamellae, through which theheating steam is caused to flow, with the liquor being caused to flow onthe outside as a falling film. In this way, an open communication isobtained on the liquor side transversely to the direction of flow of theliquor. The heating by means of the heating surface evaporates theliquor, which is removed at the bottom part of the effect, with a partof the liquor being circulated and a part being conveyed onwards to thenext effect.

In certain evaporators possessing vertical pipes, the liquor is causedto flow inside the pipes and the steam to surround the pipes, instead ofthe other way round, with this consequently giving rise to a closedcommunication on the liquor side transversely to the direction of flowof the liquor.

In falling-film evaporations, all the evaporation from the film broadlyspeaking takes place instantaneously. SE-C-504232 presents afalling-film evaporator in which the liquor flows on the outside ofhorizontal pipes, which pipes are heated by aqueous steam. As the liquorfilm falls downwards, it is concentrated, with the emission of liquorvapour, which liquor vapour is led away via an outlet. In this case, theliquor is therefore flashed (decompressed) in the evaporator. A similarfalling-film evaporator is also presented in SE-C-512959, where thisfalling-film evaporator has been provided with an improved system fordistributing the aqueous steam to the pipes. In both these evaporators,the concentration of the liquor is increased successively in thefalling-film evaporator. An improved falling-film evaporator ispresented in U.S. Pat. No. 5,624,531, in which the pipes are arrangedvertically instead, whereupon the black liquor forms a film on theoutside of the pipes. This type of evaporator has been installedextensively in black liquor-evaporation lines and is marketed byKvaerner Pulping AB under the trade name TUBEL™. In this case, too,evaporation from the liquor film takes place continuously and liquorsteam is extracted via the upper outlet at the top.Evaporation-concentrated black liquor is withdrawn at the bottom.

A type of evaporator is also what is termed a forced-circulationevaporator in which a liquor stream is first heated up in vertical orhorizontal tubular heat exchangers, with the liquor in closedcommunication on the inner side of the tubes, thereby causing thetemperature of the liquor to increase. The actual evaporation only takesplace, by means of flashing, in a subsequent expansion tank having alower steam pressure.

U.S. Pat. No. 4,857,146 presents a final evaporation stage in which theliquor is conducted, in a pressurized state, to a heat exchanger suchthat no boiling takes place in the heat exchanger. Only after theheating in the heat exchanger is the pressure lowered in a decompressiontank (flash tank) to a level below the prevailing saturation pressure ofthe liquor. In this case, use is made of a conventional heat exchangerin which the liquor side is completely filled.

U.S. Pat. No. 5,112,441 presents a combination of a conventionalevaporation stage in which evaporation down to 65% dry substance contenttakes place in the evaporation stage (the concentrator), with steambeing blown off from the stage, with this being followed by a subsequentdecompression of the liquor in a flash tank down to a lower, butnevertheless pressurized, level. In this case, evaporation thus takesplace in two stages, firstly in the evaporator and then in a flash tank,but nevertheless while retaining a certain pressure.

A number of other different constructions and plants which are based onthese principles are well known within the technical field.

THE TECHNICAL PROBLEM

Since the liquid which is intended for evaporation contains a largeproportion of solid and dissolved substances, the problem exists thatsome of the contents of the liquid can become deposited on the walls inor on the pipes or lamellae. This so-called incrustation usually arisesas a result of the crystallization of dissolved salts on the heatingsurfaces. An aggravating factor is when so-called nuclear boiling takesplace directly in association with the heating surface, since thisboiling can give rise locally to very high concentrations of solid ordissolved substances.

No nuclear boiling, or only very little nuclear boiling, will take placeat the heating surfaces in forced-circulation appliances havingliquid/liquor sides which are completely filled with liquid/liquor andin which all the evaporation takes place in a subsequent separationspace. This a positive factor from the point of view of incrustation.However, these appliances have a limited capacity to increase thetemperature of the liquor when the liquor volumes are relatively largein relation to the heating surfaces.

In the falling-film technique, the heating surface and the separationspace are constructed as one unit, which provides a very efficientconstruction, which is also characterized by a very efficient use of theheating surfaces due to its high heat transfer values even at a lowerelectricity consumption than in the case of the forced-circulationtechnique. In the falling-film technique, the temperature of the liquorcan be increased rapidly since the liquor volumes are relatively smallin relation to the heating surfaces. However, in a falling-filmevaporator, it is not possible to avoid nuclear boiling at the heatingsurface in association with a relatively high heat load, something whichmakes this technique more susceptible to incrustation than is theforced-circulation technique.

That which is most characteristic of the falling-film heat exchanger isthat the film constitutes a relatively small part of the volume on theliquid side, i.e. substantially less than 40% of the total volume on theliquid side, normally less than 20% of the volume. The black liquorforms a thin film, typically 1-2 mm thick, on the heating surfaces. Inthis way, the volumes of the liquid which have to be heated can bereduced relative to the heat-transferring surfaces, resulting in thefilm on the heating surfaces being heated rapidly. It is this effectwhich has hitherto been exploited in falling-film evaporators for blackliquor, in which the black liquor undergoes rapid evaporation as thefilm of liquor runs down over the heating surfaces. The remaining partof the total volume on the liquid side is taken up by gas phase(including liquor vapour).

The consequences of a possible incrustation are very serious both in thecase of forced-circulation evaporation and in the case of falling-filmevaporation in conventional tube appliances, both of which operate withthe liquor in closed communication on the inside of the tubes, since aplugged tube cannot be washed clean during operation by means of simplylowering the concentration; instead, the plugged appliance has to beshut down for mechanical cleaning, resulting in a disturbance ofproduction. In modern falling-film evaporators of the lamella type or ofthe tube element type (with the falling film on the outside of thetubes), both of which have open communication on the liquor side, thisproblem is counteracted by arranging for the concentration to be loweredduring ongoing operation, a procedure which will dissolve anyincrustation which has formed.

By means of its construction, with the heating steam inside the tubes, afalling-film evaporator of the tube element type is a favourablemechanical construction, especially at relatively high steam pressures.

While the falling-film technique affords a more efficient evaporationappliance, with lower electrical energy consumption and better washingcapacity, it has a higher rate of incrustation than does theforced-circulation technique, at least at relatively high heat loading.

THE SOLUTION

That which is characteristic of the invention is that use is made of afalling-film heat exchanger in which, in contrast to known technique,boiling, and thus continuous evaporation in the thin film in thefalling-film heat exchanger, is avoided. By means of pressurizing theliquor vapour on the liquor side to a pressure which is well above thesaturation pressure for the liquor at its prevailing temperature, theboiling can be efficiently suppressed and the incrustation problemsavoided.

According to the present invention, a process for evaporating liquids,for example black liquor from cellulose cooking, containing solid anddissolved substances which are to be concentrated has been developed forthe purpose of overcoming the above-mentioned problems, in which processthe liquid in a falling-film heat exchanger of, for example, the tube oflamella type is heated to the vicinity of the boiling temperature. Theinvention also relates to a device for evaporating liquids.

According to the invention, use if preferably made of a falling-filmtechnique of the tube element type (TUBEL™) or lamella type, both ofwhich have the falling liquor film on the outside of the tubes orlamellae.

According to the invention, preference is given to the pressure on theliquid vapour on the liquid side in the falling-film heat exchangercontinuously corresponding to, or exceeding, the saturation pressure forthe liquid when operation has become established, and corresponding to,or exceeding, the saturation pressure for the water in the liquid duringblack liquor evaporation, and, preferably, to this being more than 0.1bar above the saturation pressure for the liquid, i.e. the pressure atwhich the liquid would boil at the highest temperature prevailing in thefalling-film heat exchanger, preferably at least 0.5 bar above thesaturation pressure. There should be substantially no continuousextraction of liquid vapour from the liquid side. This makes it possibleto efficiently suppress nuclear boiling in the black liquor in thefalling-film heat exchanger.

According to the invention, the liquid is heated using a heating mediumin the form of steam under pressure.

According to a preferred example of the present invention, the pressureon the heating side in the falling-film heat exchanger is approx. 3.6bar and the temperature is approx. 140° C., while the pressure on theliquid vapour on the liquid side in the falling-film heat exchanger isapprox. 2 bar and the temperature 134° C., with the pressure in theexpansion tank being approx. 1.5 bar, with the temperature of the liquidbeing cooled down to approx. 130° C. during intense flashing.

DESCRIPTION OF THE FIGURES

The invention will be described in more detail below with reference tothe attached figures in which

FIG. 1 shows an outline diagram of an appliance which is used inassociation with the present invention;

FIG. 2 shows a more detailed variant of FIG. 1, and

FIG. 3 shows how the liquid is distributed as a film in the falling-filmheat exchanger.

DETAILED DESCRIPTION

FIG. 1 shows a falling-film evaporation appliance, i.e. what is termedan effect 1, of conventional construction. This effect can possesslamellae or pipes in its interior, through or on which the liquid whichis intended for evaporation flows. The liquid is fed in via the pipe 2,and this liquid principally consists of previously evaporated,circulating liquid. Some of this liquid, which may be ready forcombusting or which is to be fed to another effect, is withdrawn throughpipe 3. The circulation is brought about using a pump 4. The heatingmedium, i.e. the steam which is to impart to the liquid, which entersthrough the pipe 2, the requisite temperature for evaporation, is fed inthrough the pipe 5. While this steam can be what is termed primarysteam, it can also come from a preceding effect.

The liquid which has been caused to flow through the effect 1 is removedat the bottom of the effect through the pipe 6 and fed through apressure reduction valve 10 into the expansion vessel 7, which is at alower pressure than that which corresponds to the pressure at which theliquid coming through the pipe 6 boils. Intensive boiling will thereforetake place in the expansion tank 7, with the liquid being cooled downand vapour being formed, which vapour is removed through the pipe 8. Theliquid which has been concentrated by evaporation is removed from thelower part of the expansion tank 7 and caused to circulate through thepipe 2 or conducted out through the pipe 3. Fresh liquid which is to beconcentrated is fed through the pipe 9 into the expansion vessel 7 andcaused to mix with the liquid which has been concentrated by evaporationin the expansion tank 7.

The figure only shows the apparatus which is required in accordance withthe present invention diagrammatically. The invention can be applied toall the effects in a plant, if so desired, and these effects can be ofany type whatsoever. That which is important is that nuclear boiling isprevented in the effect 1 and that the evaporation as such takes placein the expansion tank 7.

FIG. 2 shows an advantageous variant in which the components/parts whichhave a corresponding function to those shown in FIG. 1 have the samereference number. In this figure, the falling-film heat exchanger 1 isshown with its assembly of vertical tubes 22. A heating medium, i.e.heating steam HS, flows through the tubes, i.e. via the inlet 5, andthence to distribution boxes and out into the tubes 22, and residualsteam CS is ventilated via the outlet 24. Condensate C which has formedis tapped off from the lowest point in the system. The constructionresembles that which is presented in U.S. Pat. No. 5,624,531.

The liquor which is to be heated on the liquid side is fed in via anelement 21 which is placed at a high level, for distributing the liquid.The distribution element can consist of a box with holes in the bottomfrom which the liquid drops down over the tube assembly. Theliquid/liquor then forms a falling film of liquid over heating surfaces,which are heated by the heating medium, on the outside of the tubes. Theway in which this film is formed is shown in FIG. 3, in which a numberof tubes, through which heating media SF flow, are seen in crosssection, and in which the film LF is formed on the outside of the tubes,on a liquid side which is otherwise filled with a liquid vapour GF.

If there is a need to be able to establish the requisite pressure on theliquid vapour on the liquid side already at start-up, means are thenrequired for supplying pressurized steam. This is obtained via a feedline 30 for pressurizing steam HS, expediently the same steam as usedfor the heating medium, which steam pressurizes the liquid vapour phaseon the liquid side via a pressure control valve 30 a. By means ofsupplying pressure in this way, it is possible to already establish therequisite pressure on the liquid side when starting up from thefalling-film heat exchanger being in a cold state. In certainapplications in which the equipment is run continuously, thepressurization can in this way be dispensed with since the requisitepressure is built up during start-up and, when the operation has becomeestablished (what is termed the steady state), this pressure is thenmaintained due to the heating medium being supplied continuously.

While the falling-film heat exchanger is not intended to give rise toany boiling of the liquid on the liquid side, temperatures which are toohigh can be obtained on the heating side in connection with certainoperational states, a situation which can give rise to pressures whichare too high. If the liquid contains other non-condensable gases whichare more readily volatile than water, for example air or nitrogen, acertain overpressure can also be established. It is possible, via adecompression valve 31 a in an outlet line 31, to ventilate away anypossible overpressure intermittently from the liquid vapour phase on theliquid side to the decompression tank 7, expediently by way of an inlet34 above the liquid level 43 in the tank 7. Due to the equilibriumexisting between water in the liquid vapour phase on the liquid side andthe liquid phase, some water will always be present in the liquid vapourphase. Even if some of the liquid vapour phase has to be ventilated dueto the overpressure which has developed, only a negligibly small part ofthe water will accompany this ventilated vapour, i.e. substantially lessthan 1% of the water content of the integral black liquor, and typicallyof the order of size of a few parts per thousand of the water content inthe integral black liquor. Consequently, substantially more than 97%,typically more than 99%, of the water content in the integral blackliquor should be retained by the falling-film heat exchanger. The liquidwhich is pressurized and heated in the falling-film heat exchanger isconducted to the decompression tank 7, by way of the pipe 6, thepressure reduction valve 10 and an inlet 41, which inlet is arrangedabove the water level 43 formed in the tank, with decompression of theliquid taking place in the gas phase. The pressure reduction valve 10 iscontrolled using a liquid level transmitter 42 such that a certainliquid level is maintained in the falling-film heat exchanger or atleast in the falling-film heat exchanger's outlet pipe 6 for thepressurized and heated liquid. The liquid level is established at alevel which is suitable for ensuring that there is no risk of thepressure in the falling-film heat exchanger blowing out in the tank 7.

Fresh liquid which is to be evaporated is conducted into the system viathe decompression tank 7, by way of the pipe 9 and an inlet 40 which isarranged above the liquid level 43 which is formed in the tank, with thefresh liquid, which is at a lower temperature and a lower concentration,firstly being heated in the gas phase of the decompression tank by meansof direct heat exchange. Expediently, the fresh liquid is conveyed intothe gas phase by way of some type of spray/atomizer (not shown). Thisresults in a first heating of the fresh liquid in the gas phase of thetank, with a first degree of incrustation thereby being obtained in amanner which is directed towards the tank 7, and not being obtained inthe falling-film heat exchanger. What is substantially a deliberateformation of any possible incrustations is obtained in the tank 7,thereby considerably reducing the risk of incrustation formation in thefalling-film heat exchanger.

In order for the present invention to have a particularly good effect,it is possible to increase the rate of circulation very substantially ascompared to what is usual in a conventional falling-film appliance, i.e.to what is two to five times the normal rate.

An example of the actual conditions in the appliance, which can be afalling-film evaporator with liquid flowing on the outside of the pipes,is that the pressure of the steam which enters through the pipe 5 isapprox. 3.6 bar, while its temperature is 140° C. In this connection,the pressure on the liquid which is entering through the pipe 2 andwhich is caused to flow on the outside of the vertical pipes in theeffect 1 can be approx. 2 bar while the liquid has a temperature ofapprox. 134° C. This liquid, which is to be partially volatilized in theexpansion tank 7, is admitted into this tank against a steam pressure ofapprox. 1.5 bar, that is a pressure which is approx. 0.5 bar lower thanin the effect 1, with vapour being released rapidly while the liquidcools down to approx. 130° C. The abovementioned approximate figures forpressure and temperature are also intended to include deviations withinthe range +/−10%.

Apart from saving the costs involved in repeated interruptions toproduction, an advantage of the present invention is that the heatingareas can be made much smaller since there is no need to allow forimpurities. This represents a substantial economy. The invention alsomakes it possible to have fewer sections and fewer circulation rounds,with this too leading to economies as a result of fewer instruments,pumps and control valves. In certain cases, automatic washing valves canalso be dispensed with.

While the pressure difference between the pressure in the expansion tank7 and the liquid side in the effect 1 is expediently less than 1 bar, anupper limit is only specified for practical reasons.

The invention is not limited to the embodiment shown; on the contrary,it can be varied in different ways within the scope of the patentclaims. For example, the pressurizing 30, 30 a can be omitted if thefalling-film heat exchanger is essentially run continuously and thestart-up procedure takes place rapidly. The pressure difference betweenthe liquid side and the decompression tank 7 can also be obtained usinga fixed throttle 10, with the difference in height H being establishedfor the purpose of obtaining a suitable pressure difference and thethrottle being adjusted so that no boil-off takes place in the pipe 6.Any overpressure in the falling-film heat exchanger can also beregulated by controlling the flow of the heating medium HS. In thatcase, any possible overpressure valves can only be introduced as anadditional secondary safety measure, where the primary control takesplace by way of the flow of heating medium.

1. A process for evaporating a liquid from cellulose cooking, comprising: heating the liquid in an indirect falling-film heat exchanger, the heat exchanger having a heating side and a liquid side, the heating side being entirely filled with a heating medium; distributing the liquid on the liquid side over heating surfaces that are being heated by the heating medium; the liquid disposed on the heating surfaces forming a thin film over the heating surfaces, the thin film of liquid occupying less than 40% of a volume on the liquid side, filling a remainder of the volume on the liquid side with a vapor; maintaining the vapor with a first pressure that is sufficiently high to prevent boiling of the liquid on the liquid side; transferring the pressurized liquid to a separate expansion tank; providing the expansion tank with a second pressure that is lower than the first pressure; and permitting the liquid disposed in the expansion tank to evaporate.
 2. The process according to claim 1 wherein the process further comprises providing the heat exchanger with tubular elements.
 3. The process according to claim 1 wherein the process further comprises providing the heat exchanger with lamellar elements.
 4. The process according to claim 1 wherein the process further comprises maintaining the first pressure at or above a saturation pressure for the liquid to prevent boiling of the liquid.
 5. The process according to claim 4 wherein the process further comprises maintaining the first pressure more than 0.1 bar above the saturation pressure.
 6. The process according to claim 4 wherein the process further comprises maintaining the first pressure more than 0.5 bar above the saturation pressure.
 7. The process according to claim 1 wherein the process further comprises heating the liquid with steam under pressure.
 8. The process according to claim 1 wherein the process further comprises providing the heating medium with a pressure of about 3.6 bar and a temperature of about 140° C. and maintaining the first pressure of the liquid vapor at about 2 bar and a temperature of about 134° C. and maintaining the second pressure in the expansion tank at a pressure of about 1.5 bar and cooling the liquid down to a temperature of about 130° C.
 9. A device for evaporating a liquid, comprising: an indirect heat exchanger having a heating side and a liquid side; the heating side having an inlet defined therein for a heating medium and an outlet defined therein for a heating medium; the liquid side being provided with a distributing element being disposed at a high level on the liquid side for distributing the liquid; the distributing element being arranged to distribute the liquid as a falling film over heating surfaces that are being heated by the heating medium; the falling film taking up less than 40% of a total volume of the liquid side; the liquid side of the heat exchanger having an outlet conduit being disposed at a low level that is below the high level of the distributing element; the outlet conduit being filled with the liquid; a remainder of the total volume of the side being filled with a vapor; pressure means for pressurizing the vapor on the liquid side to a pressure exceeding a saturation pressure of the liquid on the liquid side to suppress a boiling of the liquid; a throttle in operative engagement with the outlet conduit; and a decompression tank connected to the outlet conduit for permitting the liquid to boil and evaporate in the tank by decompression via the throttle.
 10. The device according to claim 9 wherein a feed line is connected to the heat exchanger for providing a heating steam to pressurize the vapor and a pressure control valve is in operative engagement with the feed line.
 11. The device according to claim 9 wherein an outlet line is connected to the heat exchanger and a decompression valve is in operative engagement with the outlet line to ventilate overpressure from the heat exchanger to the decompression tank.
 12. The device according to claim 9 wherein the device has means for maintaining a liquid level in the decompression tank and the decompression tank has an inlet defined therein, the inlet being disposed above the liquid level.
 13. The device according to claim 12 wherein the decompression tank has an inlet defined therein, the inlet being located above the liquid level in the decompression tank.
 14. A process for evaporating a liquid from cellulose cooking, comprising: feeding the liquid to an indirect falling film heat exchanger; heating the liquid in the indirect falling-film heat exchanger, the heat exchanger having a heating side and a liquid side, the heating side being entirely filled with a heating medium; distributing the liquid on the liquid side over heating surfaces that are being heated by the heating medium; the liquid disposed on the heating surfaces forming a thin film over the heating surfaces, the thin film of liquid occupying less than 40% of a volume on the liquid side, filling a remainder of the volume on the liquid side with a vapor; maintaining the vapor with a first pressure that is sufficiently high to prevent boiling of the liquid on the liquid side; transferring the pressurized liquid to a separate expansion tank; providing the expansion tank with a second pressure that is lower than the first pressure; permitting the liquid disposed in the expansion tank to evaporate; circulating a first portion of the evaporated liquid in a circulation conduit from the expansion tank back to the indirect falling film heat exchanger; and withdrawing a second portion of the evaporated liquid to an evaporation effect unit.
 15. A process for evaporating a fresh liquid from cellulose cooking, comprising: feeding the fresh liquid to be evaporated to an expansion tank having a liquid disposed therein; mixing the fresh liquid with the liquid in the expansion tank to form a liquid mixture; feeding a portion of the liquid mixture to an indirect falling film heat exchanger; heating the liquid mixture in the indirect falling-film heat exchanger, the heat exchanger having a heating side and a liquid side, the heating side being entirely filled with a heating medium; distributing the liquid mixture on the liquid side over heating surfaces that are being heated by the heating medium; the liquid mixture disposed on the heating surfaces forming a thin film over the heating surfaces, the thin film of liquid occupying less than 40% of a volume on the liquid side, filling a remainder of the volume on the liquid side with a vapor; maintaining the vapor with a first pressure that is sufficiently high to prevent boiling of the liquid mixture on the liquid side; transferring the heated and pressurized liquid mixture to the expansion tank; providing the expansion tank with a second pressure that is lower than the first pressure; permitting the pressurized liquid mixture to expand and evaporate in the expansion tank; circulating a first portion of the liquid mixture in a circulation conduit from the expansion tank back to the indirect falling film heat exchanger; and withdrawing a second portion of the liquid mixture to an evaporation effect unit. 